The present invention relates generally to laser hardened materials and structures, and more particularly to a novel optical filter structure for selectively blocking laser radiation of predetermined wavelength while passing radiation of other wavelengths.
The present invention provides a substantially improved optical reflection band filter which becomes reflective upon absorption of a certain amount of radiation by a laser beam of intensity I and wavelength .lambda..sub.0. The invention is constructed by immersing a crystalline array of microspheres of optical dimensions within a nonlinear optical medium which fills the void between the microspheres of the crystalline array. The refractive index of the nonlinear medium is matched to the refractive index of the microspheres in the absence of laser irradiation, producing optical (i.e., relating to wavelengths within the optical spectra including the visible and near and mid infrared from about 0.3 to 2 microns, within which range the filter of the invention best functions) transparency at all wavelengths. Alternatively, the microspheres may be nonlinear in a linear medium, or the microspheres and medium may have diverging nonlinearity with incident electric field In these instances, laser irradiation causes an index mismatch between the microspheres and the surrounding medium, producing Bragg diffraction of laser light by the planes of the crystalline array. The filter structure defined by the teachings herein greatly enhances the efficiency of the inhomogeneous media Kerr effect filters through coherent diffraction of radiation rather than through incoherent scattering of radiation. The filter blocks laser radiation of selected characteristic wavelength .lambda..sub.0 while allowing other wavelengths to pass. An optical system protected by the invention may therefore receive a broad spectrum of optical signals while being protected from an invasive, potentially destructive laser beam. In the absence of intense invasive laser radiation the filter is nominally transparent to all wavelengths. The invention demonstrates numerous advantages over standard quarter-wave interference stack filters, rugate interference filters, noncrystalline inhomogeneous media Kerr effect filters, linear crystalline array microsphere filters, and other wavelength responsive laser hardening filters. The filter structure taught herein may therefore find substantial utility in safety goggles, narrow band optical rejection filters, optical switches and limiters for signal and data processing applications and like optical devices and systems.
It is, therefor principal object of the invention to provide an improved optical filter.
It is a further object to provide an optical filter for blocking laser radiation of a preselected wavelength while passing radiation of other wavelengths.
It is yet another object to provide a switchable optical filter having fast response time.
It is a further object to provide a filter having a large absorbance value at the design wavelength.
It is a further object to provide an optical filter which is switchable at preselected impinging electrical field, has low absorption losses prior to switching, and is independent of impinging radiation prior to switching.
It is yet a further object to provide an optical switch having much greater efficiency than inhomogeneous media nonlinear Kerr effect filters.
These and other objects of the invention will become apparent as the description of representative embodiments proceeds.